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  UNIT. 5 (ground improvement)

Stone columns:

  Stone columns refer to columns of compacted, gravel size stone particles constructed vertically in the ground to improve the performance of

soft or loose soils. The stone can be compacted with impact methods, such as with a falling weight or an impact compactor or with a vibroflot, the more

common method. The method is used to increase bearing capacity (up to 5 to 10 sf or !"0 to "#0 $a%, reduce foundation settlements, improve slope

stability, reduce seismic subsidence, reduce lateral spreading and li&uefaction potential, permit construction on loose'soft fills, and precollapse sinholes prior 

to construction in arst regions.

Design Considerations for stone columns:The design of stone columns is still an empirical process however, general design guidelines have been developed and are provided below.

  )or stone columns to ade&uately perform, the soils surrounding the columns must provide sufficient lateral support to prevent bulging failures. *naddition, the columns should terminate in a dense formation to prevent bearing failures. Stone columns are typically stiffer than the materials that surround

the columns therefore, the columns will settle less and will carry a larger portion of the applied load. The applied load is transferred between columns

through soil arching. +ltimately e&uilibrium is reached when sufficient load has been transferred to the columns to prevent further settlement of the

surrounding soils. *n stability and bearing analyses, composite shear strength of the soilstone column matri- is used. The composite shear strength is based

on the shear strength of the insitu soils, the shear strength of the stone materials, the area replacement, and stress ratios.Unit Cell Concept

ccording to /round *mprovement ethods, )or purposes of settlement and stability analyses, it is convenient to associate the tributary area of soil

surrounding each stone column with the column il lustrated in )igures 19-13 and 19-14. lthough the tributary area forms a regular he-agon about the stone

column, it can be closely appro-imated as an e&uivalent circle having the same total area. The resulting e&uivalent cylinder of material having a diameter

(2e% enclosing the tributary soil and one stone column is nown as the unit cell3. The stone column is concentric to the e-terior boundary of the unit cell.

)igure 14.1 stone column e&uilateral triangular pattern

)igure 14.1" unit cell idealization

 Area Replacement Ratio

The Area Replacement Ratio ( 6s) defines the area of the soil replaced by the stone column as a function of the tributary area of theunit cell to the area of the stone column. The more soil replaced by the stone column, the greater the effect on performance. Typical values of 6s range from 0.10 to 0.40. 

6s  As ! A

as 1 ! 6s

"here,

6s  Area replacement ratio  As  Area of the stone column  A Total area #ithin the unit cell  as  Area improvement ratio

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 Spacing and Diameter

ccording to /round *mprovement ethods, Stone column diameters vary between 1.5 and ".0 feet, but are typically in the range of .0 to .5

feet for the dry method of installation, and somewhat larger for the wet method of installation.

Triangular, s&uare or rectangular grid patterns are used with centertocenter column spacing of 5.0 to 11.5 feet. )or footing support, the stone columns are

installed in rows or clusters. )or both footing or wide area support, the stone columns may e-tend beyond the loaded area.3

 Stress Ratio

The transfer of the applied load to the stone columns from the insitu soils depends on the relative stiffness of the stone column to the insitu soils, aswell as the spacing and diameter of the stone columns. 7ecause the stone columns and the insitu soils deflect (strain% appro-imately e&ually, the stone

columns must be carrying a greater portion of the load (stress% than the insitu soils. This concept has also been called the e&ual strain assumption. This

concept has been proven by both field measurements, as well as finite element analysis. The relationship between the stress in the stone column and the stress

in the insitu soil is defined in the following e&uation.

 n 8s ! 8c

9here,

n : Stress ratio or stress concentration 

8s : Stress in the stone column

8c = Stress in the surrounding soil

easured values of n have generally been between !.0 and 5.0. The theory indicates that n should increase with time. high nvalue ( to "% may be re&uired

in very wea soils and the column spacing is tight. ;ower values of n (! to !.5% are re&uired when the surrounding soil is stronger and the column spacing is

wider. )or preliminary design, a conservative nvalue of !.5 should be assumed.

Verification

ccording to /round *mprovement ethods, *nsitu testing to evaluate the effect of the stone column construction on the native cohesive soil can be also specified. etting water is used to advance the vibroflot, the e&uipment and setup is similar to ?=. *f >etting water is not desired for a particular

 pro>ect, the dry bottom feed process can be used. tremie pipe, through which stone is fed to the tip of the vibroflot, is fastened to the side of the vibroflot.

stone sip is filled with stone on the ground with a front end loader and a separate cable raises the sip to a chamber at the top

of the tremie pipe.  specific application is referred to as vibro piers. The process refers to short, closely spaced stone columns designed to create a stiff bloc to

increase bearing capacity and reduce settlement to acceptable values. ?ibro piers are typically constructed in cohesive soils in which a full depth predrill hole

will stay open. The stone is compacted in 1 to ! ft (0." to 0.# m% lifts, each of which is rammed and compacted with the vibroflot.

 Procedure:

The column construction starts at the bottom of the treatment depth and proceeds to the surface. The vibrator penetrates into the ground, assisted by

its weight, vibration, and typically water >ets in its tip, the wet top feed method. *f difficult penetration is encountered, predrilling through the firm soils may

also be performed. front end loader places stone around the vibroflot at the ground surface and the stone falls to the tip of the vibroflot through the flushing

water around the e-terior of the vibroflot. The vibrator is then raised a couple of feet and the stone falls around the vibroflot to the tip, filling the cavityformed as the vibroflot is raised.

The vibroflot is then repeatedly raised and lowered as it is e-tracted, compacting and displacing the stone in ! to ft (0.@5 to 0.4 m% lifts. The flushing

water is usually directed to a settlement pond where the suspended soil fines are allowed to settle. *f the dry bottom feed procedure is selected, the vibroflot

 penetrates into the ground, assisted by its weight and vibrations alone ()igure 1!.11b%. gain, predrilling may be used if necessary or desired. The remaining

 procedure is then similar e-cept that the stone is feed to the tip of the vibroflot though the tremie pipe. Treatment depth as deep as 100 ft (0 m% has beenachieved.

figure. Installation of stone columns

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Uses of stone columns:

The stone column techni&ue of ground treatment has proven successful in

(1% improving slope stability of both eubanments and natural slopes,

(!% increasing bearing capacity,

(% reducing total and differential settlements,

("% reducing the li&uefaction potenABial of sands and

(5% increasing the time rate of settlement.  Stone columns are used to support structures overlying both very soft to firm cohesive soils and also loose silty sands having greater than about 15

 percent fines. t the present time, more stone column pro>ects in the +.S. have been constructed in silty sands rather than cohesive soils worldwide the

reverse is true.

ApplicationsStone columns have been used successfully in the +.S. before 14#! on !1 pro>ects including the following applications CD#EB

1. Fmbanment )ill Support for highways, interchanges and bridge approaches.!. iscellaneous

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Design of lime and lime/cement columns, Fsi > !"

  The bearing capacity and the undrained shear strength of soil stabilized by lime and lime'cement are usually estimated from the pea

resistance of the columns and of the unstabilized soil between the columns when the shear strength of the columns is less than 100 to 150 $a and there&uired increase of the stability is small. +nder favourable conditions a characteristic shear strength of 150 $a can be used in design, when the global factor 

of safety without the columns is larger than 1.! ()s K 1.!%. n effective stress analysis is used to evaluate the longterm bearing capacity with L0

col of 0L for lime columns, 5L for lime'cement columns and "0L for cement columns. The pore water pressure in the columns ucol

could then be as high as the pore water pressure in the surrounding unstabilized soil. circular slip or failure surface through the columns is normally

used to determine the factor of safety. The load distribution between the columns and the unstabilized soil corresponds to the modulus of elasticity for thecolumns and to the compression modulus for the unstabilized soil, Fcol and soil, respectively. The re&uired global factor of safety is 1.5 according to the

Swedish Hoad 7oard and the Swedish Gational Hail dministration. Single or double rows with overlapping columns are usually re&uired to

stabilize slopes, e-cavations and embanments. The re&uired overlap is 50mm for columns with 0.Dm diameter. The individual columns should

e-tend into a layer with a high bearing capacity to prevent failure along a slip surface below the columns.Design of lime and lime/cement columns, Fsi # !"

  =olumns with an unconfined compressive strength e-ceeding !00M00 $a are used to increase the stability when the initial factor of safety without the

columns is less than 1.!. *t is proposed that the stability could be evaluated from the unconfined compressive strength of the columns up to 1.0$a,

when a large increase of the stability is re&uired, by considering the reduction of the stability of the columns and of the

unstabilized soil by progressive failure

. by locating the columns in the active zone below an embanment or a fill

. by designing the columns to carry the full weight of the embanment or the fill

. by ensuring that the weight of the embanment or the fill is transferred to and from the columns without e-cessive settlements

. by replacing the soft or loose soil above the columns and the wea upper part of the columns by compacted granular material

. by resisting the lateral earth pressure in the embanment by geofabric or geoanchors

. by checing that the columns do not contain wea layers or lenses which could reduce the bearing capacity of the columns.  *t is proposed to calculate the settlements with a modulus of elasticity of 

150&u,col up to an e&uivalent preconsolidation pressure p0 c N 1B@&ucol and a ma-imum creep strength of 1.0 $a.

Limitations of $resently used design met%ods

  Slope and bearing capacity failures have occurred, which indicate that the stability could be overestimated for single columns, when an average

shear strength is used to estimate the stability of slopes and of embanments. Fven a small lateral displacement could fail the columns when the shear

strength of the columns is high and the ductility is low. 0.Dmdiameter lime'cement and cement column could fail when the lateral displacement is only !0 to0mm. The shear resistance of column rows could be less than the pea shear strength. The shear resistance can also be low for floating column rows

when the slip surface is located >ust below a column row with pointbearing columns. The column rows could also fai l by overturning, translation, separation

or by internal shear along the overlap at the centre of the column row.

&otal and effective stress analysis

  The bearing capacity and the shear resistance of lime, lime'cement and cement columns are governed by the drained shear strength,depending on the loading rate and the permeability of the columns. The undrained shear strength usually governs the shear strength of the unstabilized soil.

The pore

water pressure in the unstabilized soil around the columns could govern the shear resistance of lime'cement columns in an effective stress analysis if 

calculations or measurements donOt indicate otherwise since the hydraulic lag can be large in the columns. The shear strength of lime'cement and cementcolumns increases with time as the e-cess pore water pressure in the columns dissipates during the consolidation. The lowest shear strength is e-pected >ust

 below the dry crust where the effective overburden pressures and the shear strength of the unstabilized soil are low.

  The undrained shear strength, Lfu,col of the soil stabilized by lime'cement or cement, which governs the stability of embanments, slopes,

trenches and e-cavations, increases with increasing confining pressure when the stress level is low. The drained shear strength is evaluated by the following

e&uation.(#.!"%where Ld,col is the drained angle of internal friction of the stabilized soil, which varies with soil type, the confining pressure and with the water 

content, cd,col is the drained cohesion and Lf is the total normal pressure on the failure plane through the columns. The shear strength usually increaseswith increasing clay content and with increasing plasticity inde-.The shortterm bearing capacity depends on the confining pressure Lh,

which can be estimated by the following e&uation.(#.!5%

where Lho is the initial total lateral earth pressure, cu,soil is the undrained shear strength of the unstabilized soil, Fsoil, is the modulus of elasticity and Lsoil is $oissonOs ratio. The term Jsoilmo,soil &o is the increase of the effective lateral earth pressure caused by the applied unit load &o, mo,soil, is the stress

factor for the unstabilized soil and Jsoil is the coefficient of lateral earth pressure, which is assumed to be at least 1.0 due to the volume increase

during the slaing of the lime. t Fu,soilN!00cu,soil and LsoilN0.5

'lobal factor of safety

The variation of the shear strength of the stabilized soil is often large across the diameter and along the length of the columns. *t is therefore difficult to

determine a characteristic shear strength and a characteristic compression modulus by unconfined compression or by tria-ial tests. single global factor ofsafety, )sN!.0 to .0, is generally used in Sweden to determine the allowable load. minimum factor of safety of 1.! to 1.5 is usually re&uired for

embanments and slopes, respectively, for both short and longterm conditions. The factor of safety should be at least 1.5 according to the Swedish Hoad

7oard .The re&uired factor of safety with respect to bottom heave is 1..The bearing capacity of buildings and of other structures, which are

supported by lime or lime'cement columns as well as the stability of embanments, deep e-cavations and shallow trenches are calculated by an

effective stress analysis for the columns and by an undrained analysis for the unstabilized soil between the columns. The bearing capacity of the columnsas determined by an effective stress analysis depends on the pore water pressure in the columns. 2ue to the high hydraulic lag in the columns, the

 pore water pressure could be as high as the pore water pressure in the surrounding unstabilized soil. The shear resistance of the columns is

increased when the e-cess pore water pressure is reduced. high factor of safety is re&uired for column rows due to the often low

shear strength of the stabilized soil in the overlapping zone between ad>acent columns and the small contact area. The shear force between ad>acent

columns is high at the centre of a column wall, when the number of overlapping columns in the column rows is large.  *t is important that the longterm bearing capacity of the columns is sufficient. The Swedish Gational Hail dministration re&uires that the life

e-pectancy of lime and lime'cement columns should be at least 100 years. *n sand and silt the shear strength could be reduced with lime when the p

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coerdams:  cofferdam is a temporary construction method used in order to do construction in wet e-cavations. *t is installed in the wor area andwater is pumped out to e-pose the bed of the body of water so that worers can construct structural supports, perform repairs and any other 

types of wor using construction e&uipment. coffer dam is also called as caisson in some parts of world.

  9oring inside a coffer dam can be dangerous if it is not installed properly or not safely pressurized. ?arious materials are used for itsconstruction and its design must be compatible with weather conditions, waves, currents, construction e&uipment, construction methods, internal

 permanent structures and ground conditions. There are various types of cofferdams such as braced, earth type, timber crib, double walled sheet pile and

cellular which are discussed below.

  /enerally, ma>or loads imposed on cofferdams are hydrostatic forces of water and dynamic forces due to current and waves and heavy e&uipment

is used for its construction such as pile drivers, cranes with clamshell bucets, concrete pumps trucs as well as pumps for dewatering are used in theconstruction process. The effective management of e&uipment on site as well as worers is an important step in cost control and maintaining efficient

 productivity.

&*+S F FF+-D.MS

The construction process for each type is different based on whether it is used on land or in water, as illustrated in figure 1. *n general there are five types ofcoffer dam and they are as follow

7raced

 FarthType

 Timber =rib

 2ouble9alled Sheet $ile

 =ellular 

Figure

 Types of cofferdams. )or use on landB

 (a% crossbraced sheet piles (b% castinplace concrete cylinder

 (c% anchored sheet piles (d % braced vertical piles with horizontal sheeting. )or use in waterB

(e% crossbraced sheet piles ( f % earth dam ( g % tied sheet piles (h% anchored sheet piles with earth berm

(i% steel sheetpile cellular cofferdam ( j% rocfilled crib.

raced offerdams

7raced cofferdam is formed from a single wall of sheet piling. *t is constructed by driving sheet piles into the ground to form a bo- around the e-cavation site

and then this bo-3 is braced on the inside of it. *nterior is dewatering using pumps. They are primarily used for bridge piers in shallow water around 05ftdepth.

+art%0&y$e offerdams

*t is simplest type of cofferdam, consists of an earth ban with a clay core or vertical sheet piling enclosing the e-cavation. +sed for lowlevel waters withlow velocity and can be easily scoured by water rising over the top.

&imber rib offerdam

*t is one of the inds of cellulartype cofferdam. *t is first constructed on land and then floated into re&uired place. The lower portion of each cell matched

with contour of river bed. *t uses roc ballast and soil to decrease seepage and sin into place. *t is also nown as “Gravity Dam”. *n general it consists of

1!O - 1!O cells. *t is used in rapid currents or on Hocy Hiver beds. *t should be properly designed to resist two lateral forces i.e tipping'overturning and

sliding.Double01alled offerdam

*n this type of cofferdam, twoparallel rows of steel sheet piles are driven into the ground and tied together with anchors and wales then filled with soil. There

are three principle typesB

 7o-B =onsists of straight flush walls

 Semicircular cells connected by diaphragms

 =ircular cells connected with tierods or diaphragms

ellular offerdam

There are two main types of cellular cofferdam they are circular and segmental. *t can be used on a temporary or permanent basic. *n this type of cofferdam

force are resisted by the mass of the cofferdam.

COFFERDAM DESIGN CONSIDERAIONS

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The following are some of the design considerations which should be checed before the construction and during the design of cofferdam.

 Scouring or undermining by rapidly flowing water 

 Stability against overturning or tilting

 +pward forces on outside edge due to tilting

 Stability against vertical shear 

 Fffects of forces resulting fromB

 *ce, 9ave, 9ater, ctive Farth and $assive Farth $ressures

  n important consideration in the design of cofferdams is the hydraulic analysis of seepage conditions and erosion of the bottom when instreams or rivers. Significant deformations of elements may occur at different stages of construction because of the typical construction of coffer dam under

adverse conditions in a marine environment, thus it is difficult to maintain close tolerances. $rovisions must be made for deviations in dimensions so

that the finished structure may be constructed according to plan.

  2econstruction of the cofferdam must be planned and e-ecuted with the same degree of care as its installation, on a stagebystage basis. Theeffect on permanent structure due to the removal of coffer dam must be considered. 2ue to this reason, sheet piles e-tending below the permanent structureare often cut off and left in place, because their removal may affect the foundation soils ad>acent to the structure.

  9here the cofferdam structure can be built on a layer of impervious soil, the area within the cofferdam can be completely sealed off. 9here the

soils are pervious, the flow of water into the cofferdam cannot be completely stopped economically, and the water must be pumped out periodically and

sometimes continuously.

  dewatered area can be completely surrounded by a cofferdam structure or by a combination of natural earth slopes and cofferdam structure. Thetype of construction is dependent upon the depth, soil conditions, fluctuations in the water level, availability of materials, woring conditions desired inside

the cofferdam, and whether the structure is located on land or in water.

F-+S .&23' 3 FF+-D.M

cofferdam involves the interaction of the structure, soil, and water. The loads imposed include the followingB

 ection of the cofferdam but also on the drag force acting along the sides.9ith flat sheet piles, the latter may be relatively small, whereas with zpiles it may be substantial, since the current will be forming eddies behind each

indentation of profile, as shown in figure

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1ave forces

9aves acting on a cofferdam are usually due to local winds acting over a restricted fetch and hence are of short wavelength and limited to height. 9aves can

also be produced by passing boats and ships, especially in a restricted waterway.

2ce forces

These are of two types, that is the force e-erted by the e-pansion of a closedin solidly frozenover area of water surface which is called as static ice force and

the forces e-erted by the moving ice on breaup which is called as dynamic ice force.

Seismic Loads

*n most of the pro>ects, they are not considered in design of temporary structures. 7ut for very large, important, and deep cofferdams in highly seismically

active areas, seismic evaluation should be performed.

.ccidental loads

ccidental loads are the loads usually caused by construction e&uipment woring alongside the cofferdam and impacting on it under the action of waves.

Mooring forces

They are derived from two separate actions. The first is the impact of the barge and tugboats as they moor to the cofferdam or the waves are produced as they

move the barges while moored. The other force is the wind pressure on the total sail area of the barge. /ale force wind is a common occurrence along most

coasts and on large laes. The combination of high wind and waves will cause ma>or damage to the cofferdam and e&uipment if no preparation is made to

accommodate those events.Scour

Scour of the river bottom or seafloor along the cofferdam may tae place due to river currents, tidal currents, or waveinduced currents. Some of the most

serious and disastrous cases have occurred when these currents have acted concurrently. very practical method of preventing scour is to deposit a blanet of 

crushed roc or heavy gravel around the cofferdam, either before or immediately after the cofferdam sheet piles are set. more sophisticated method is to lay

a mattress of filter fabric, covering it with roc to hold it in place.

 EQUIPMENS !ND M!ERI!" REQUIRED #$R INS!""!I$N 

 Equipment%s:

 $ile driving hammer 

  ?ibratory or *mpact =rane of sufficient size clamshells and draglines

 =oncrete pumps trucs

 2ewatering pumps

 7arges may be re&uired

 2ozer, loader, bachoe, trucs be may re&uired

 Materials:

 Steel sheet piles are typically used

 

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Step "

F-cavate slightly below grade, while leaving the cofferdam full of water and drive bearing piles. $lace rocfill as a leveling and support course.

Step 5B

 

Step DB =hec blocing between bracing and sheets and dewater 

 

Step @B Hemove sheet piles and bracing, as well as bacfilling and construct new structure

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'+3+-.L 3S&-U&23 M+&4D

  s we now, cofferdam is a ind of water tight construction which is designed to facilitate construction pro>ects where the particular area isnormally submerged. )or the construction of a coffer cam we have various inds of materials and e&uipmentOs which enable us to perform the

wor at a faster rate. =offerdams are rarely installed as easily as they are planned and designed. Pou must e-pect and anticipate problems that will re&uire

redesign and innovative solutions. ect.The construction of a coffer dam completely relies on following the e-act process and se&uence involved. nd also the builder and designer should possess

 proper understanding of the pro>ect. *n general, the cofferdams are limited to D0 foot long sheet piles because if these sheets are made longer than D0 foot then

it would cause difficulties in transporting, handling, threading and manufacturing.

  The first step in construction is to place the wale system after the access is wored out. The wales are placed over a barge and floated to the

 position. long with this to grip the wale system in place, guide piles and support frames are installed. 9hen the barge floods partially and towedfrom under the suspended whale frame, then using cranes the wale frame is lowered to elevation. fter that the wales are used as a guide to thread and drive

the sheet piling. Gormally at least two layers of wales are placed where the top and bottom layers will be perform as a stabilizing template to control the sheet

 piles. /enerally in marine environment we will be observing some waves, current, and wind. So to guide the sheet piles a supporting template is used as it is

almost impossible to maintain the vertical and horizontal alignment which is necessary to close the cofferdam and prevent the interlocs from splitting open.

7ut if the sheet piles are not ept plumb, then the interlocs will split apart in tension or the closing pair can bind up due to compressive friction and refuse to be driven.

  2uring cofferdam installation a driving template is used. +sually the wale system is used as a driving template. The template wales should be

mared with the proper location of every sheet pile pair interloc that touches the wale. Special care should be taen to ensure that the first pair is set plumb

in the proper location because it will be acting as a guide for the rest of the sheet piles.  *n final closure, it should never be made at a corner as this corner wors in both directions. *f either sheet wall line is out of plumb, the sheet

interloc will probably split open. The other reason to be careful in initial alignment is that this will largely define the direction the piles will tae as they

continue to penetrate the ground. *f the interloc is started off tight and out of line, it will liely split apart as it is being driven. This will damage the pile and

may re&uire very e-pensive and time consuming repair procedures. 9hen the sheet piles are fully in place and driven to the top of the upper template, the

template wales can be lowered, if needed. The pairs of sheet piles should be advanced in about five foot increments. 9ith the sheets carefully driven and thewale in position, often the sheets are welded or bolted to the top wale to provide cofferdam stability during e-cavation operations. crane and a clam bucet

usually perform the e-cavation, although in some instances a bachoe can be effective.

  F-cavation should be carried out along the sheet piles first, eeping a low hump in the middle. This allows the clam bucet to rest against the

sheets and stays upright so it can stuff the bucet. *f a depression is created in the middle of the e-cavation, the bucet will roll on its side and it

will not be able to e-cavate the wedge of soil ad>acent to the sheet piles. 9hen the e-cavation is nearly complete, a steel beam spud is placed between thewales and the sheet pile alcoves. fter the above process, tremie concreting is carried out so as to minimize the flowing concrete contact with the water. The

method is to induce the fresh concrete under the previously placed concrete and pillow it up and out. The tremie placement is a continuous operation until

completed, going !" hours a day without interruption. Tremie pours usually involve large volumes of concrete, often several thousand cubic yards of

concrete. 9hen the concrete has cured enough to gain enough strength to withstand the dewatering forces (about two or three days%, dewatering can begin.

D+1.&+-23':

  *n dewatering process, the pumping out of water from the interior of a cofferdam is carried out in such a manner that it prevents the possibility of

water moving through uncured concrete. proper sump is placed outside below the elevation of the wor which is placed and the pumped water should be

 properly discharged according to the regulations. The most important aspect during dewatering is that the underwater concrete should set so that it can

withstand hydrostatic pressure created by pumping. fter the cofferdam is dewatered, the clean Qup process can begin. The surface will be rough andundulating. There will be layers of mud, debris, and dead fish that must be cleaned up. Ince the cleanup is done, the top of the tremie concrete will have

about si- inches of laitance. The laitance is a wea layer of nearly pure cement that has been washed to the surface of the concrete by the dynamics of the

concrete tremie placement. 9hile the cleanup and laitance removal is progressing, the cofferdam will continue to lea and re&uire substantial

 pumping.

The leaage water will be contaminated by the mud and debris in the cofferdam until all remedial wor and cleanup is completed. ll waterremoved from the cofferdam during this stage probably will have to be processed before returning the water to the river, lae, or bay. t this point, a safety

 precaution is inserted. Go gaspowered machinery should ever be allowed inside a cofferdam. The danger for e-plosion and carbon mono-ide poisoning is

too great. Fven the use of diesel powered e&uipment in the cofferdam should be ept to an absolute minimum. 9henever it is possible, engines outside the

cofferdam should power all machinery. These actions will both reduce congestion in the cofferdam and provide for safer woring conditions.

 APPLICATION OF COFFER DAM:

nautical application of the term cofferdam is a watertight structure used for maing repairs below the waterline of a vessel. The name also is applied to voidtans which protect the buoyancy of a vessel. =offerdam are constructed to permit dewatering an area and facilitate the construction of foundations, bridge

 piers, dams, dry docs, and lie structures in the open air.

The following are some of its ma>or applicationsB  &'droelectric Dam Construction M =offerdams are used to divert water away from the shoreline of a river to allow for the foundations of a dam to be

constructed. *n this application, generally one half of the river width is enclosed by the cofferdam at a time to maintain overall flow.

  (ridge Construction M =offerdams are used to divert water away from bridge foundation positions, either on the shore or within the waterway.

  S)ip repair  M Sometimes cofferdams are used to generate a dry doc3 condition for a ship in order for repairs to proceed. This generally occurs when the

ship cannot be moved to an actual dry doc, and it can also be more cost effective in some cases.  $il Rig and Dam Construction M This is the primary reason why coffer dams e-ist. They are &uic to build and use welded steel and other metals they

 provide a temporary and dry platform to wor freely.

  Sun*en Vessel Reco+er': =offerdams can be used to e-pose a sunen vessel in shallow waters to allow for recovery and repair if appropriate.

  S)ip Reco+er':  very rarer use of a =offer dams is to help in recovery missions for ships that have sun in shallow water. They can be built &uicly and

aid removal in certain circumstances. *n the past coffer dams have helped recover ships such as the +SS aine, a ship which sun in 1#4# played an important part in Spanishmerican history. 7y using a coffer dam to pull up this ship from the sea bed it helped give researchers an insight

into the history of this boat.

.D5.3&.'+S F FF+-D.MS

 llow e-cavation and construction of structures in otherwise poor environment

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 $rovides safe environment to wor 

 =ontractors typically have design responsibility

 Steel sheet piles are easily installed and removed

 aterials can typically be reused on other pro>ects

D2S.D5.3&.'+S F FF+-D.MS

 Special e&uipment re&uired

 Helatively e-pensive

 Typically very time consuming R tedious

 *f rushed, sheets can be driven out of locs or out of plumb

 9hen in flowing water log >ams3 may occur creating added stress on structure

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